1. Field of the Invention
The present invention relates to a ceramic heater applicable to a glow plug for use in a diesel engine or the like, and a process for producing the same.
2. Description of the Prior Art
As conventional glow plugs, there are known ceramic heaters produced by a method wherein a heating element made of a high-melting metal such as tungsten or molybdenum is sandwiched between silicon nitride moldings, which are then hot-pressed to fire the silicon nitride portions while simultaneously integrating the silicon nitride portions with the heating section (see, for example, Japanese Patent Laid-Open No. 272,861/1994 and Japanese Patent Publication No. 19,404/1985).
The ceramic heater as disclosed in the Japanese Patent Laid-Open No. 272,861/1994, which has a heating resistor made of an inorganic conductive material and embedded in a silicon nitride sinter, is produced by producing a silicon nitride molding, disposing a coiled heating resistor made of a tungsten wire and heating resistors made of tungsten wires constituting lead wires connected to the above-mentioned heating resistor onto the silicon nitride molding, superposing thereon other silicon nitride moldings in such a way as to sandwich the heating resistors therebetween, and pressing and firing them to form a silicon nitride sinter.
It is known that a high-melting metal such as tungsten or molybdenum for forming a heater coil becomes recrystallized and brittle at a temperature of 1,100.degree. C or above. When a material filled in a protective pipe is sintered at a temperature as high as 1,400 to 1,900.degree. C. to form a ceramic heater according to the customary method, the heater coil disposed in the protective pipe becomes brittle, and this is a primary cause of disconnection of the heater coil. Furthermore, in order to sinter a slurry, an expensive sintering furnace is required while involving a complicated process. This is a primary factor of increasing the cost of the ceramic heater.